In the late 1980's to early 1990's the refrigeration and air conditioning industries switched refrigerants from R-12 (CFC-12) to R-134a (HFC-134a) due to the latter's zero ozone-depletion-potential. The mineral oil lubricants employed with R-12 were not soluble in R-134a. More polar lubricants were needed, and PAG and POE based lubricants were developed.
Because of concerns about global warming, efforts are being made to develop refrigerants that have lower global warming potential than R-134a, as well as zero ozone-depletion-potential. Indeed, R-134a cannot meet stringent newly proposed environmental standards related to global warming potential.
Much work is being done with CO2 as a refrigerant, but the operating pressures of if CO2 refrigeration systems are 5 to 10 times higher that those experienced with R-134a. These high operating pressures pose both safety and mechanical reliability concerns. Indeed, use of CO2 requires a complete redesign of refrigeration system in order to handle the elevated pressures. Thus, CO2 is not a viable ‘drop-in’ replacement for R-134a; that is, current refrigeration system cannot use CO2 as a refrigerant. The redesign expense makes CO2 an unattractive alternative to R-134a.
Difluoroethane or R-152a is another alternative refrigerant. It has a zero ozone-depletion-potential and its global warming potential is much lower than that of R-134a, which makes it attractive. However, it has not previous been pursued as a replacement for R-134a because it is mildly flammable, whereas as R-134a is essentially inert. Moreover, because of differences in physical properties, lubricants suitable for use with R-134a may not be suitable for use with R-152a. For example, lubricants have different solubilities in the two refrigerants and provide different lubricities to the refrigerants. Thus, a lubricant suitable for use with R-134a is not necessarily suitable for use with R-152a.
One known class of lubricants was described in U.S. Pat. No. 5,543,068 to Kaimai. However, the block and random copolymer and homopolymer lubricants disclosed in this patent, fail to offer sufficient solubility and/or lubricity when combined with R-134a, and thus would not likely be suitable for use in R-152a. Another class of lubricants was described in U.S. Pat. No. 6,551,523 to Schnur. However, blends of polyolesters with specific quantities of neopentylglycol esters and pentaerythritol esters, are required to achieve sufficient solubility and thermal stability when combined with R-134a. Another class of lubricants includes blends of polyalkylene glycols (PAGs) and polyolesters (POEs) with hydrofluorocarbon refrigerants (e.g. U.S. Pat. No. 4,851,144).
U.S. Pat. No. 4,959,169 to McGraw discloses the use of ester end capped random and homopolymer PAGs with R-152a, however alternative capped PAGs may be desirable. U.S. Pat. No. 5,152,926 discloses the use of blends of R-134a or R-125 with other refrigerants including R-152a, where these blends of refrigerants include a lubricant with random copolymers of PAG. R-152a by itself is not disclosed with these random copolymers. The avoidance of blends of refrigerants would be desirable to ease the manufacturing costs.
These obstacles have been significant enough to prevent the use of R-152a as a replacement. The inventors have recognized solutions to one or more of these problems.